Stress-induced bandgap-shifted semiconductor photoelectrolytic/photocatalytic/photovoltaic surface and method for making same

ABSTRACT

Titania is a semiconductor and photocatalyst that is also chemically inert. With its bandgap of 3.0, to activate the photocatalytic property of titania requires light of about 390 nm wavelength, which is in the ultra-violet, where sunlight is very low in intensity. A method and devices are disclosed wherein stress is induced and managed in a thin film of titania in order to shift and lower the bandgap energy into the longer wavelengths that are more abundant in sunlight. Applications of this stress-induced bandgap-shifted titania photocatalytic surface include photoelectrolysis for production of hydrogen gas from water, photovoltaics for production of electricity, and photocatalysis for detoxification and disinfection.

REFERENCE TO RELATED APPLICATIONS

This application is a divisional of copending application Ser. No.11/773,379, filed Jul. 3, 2007 (Publication No. 2009/0127124), whichitself is a divisional of application Ser. No. 10/424,259, filed Apr.26, 2003 (Publication No. 2003/0228727, now U.S. Pat. No. 7,485,799,issued Feb. 3, 2009), which claims benefit of Provisional ApplicationSer. No. 60/380,169, filed May 7, 2002.

BACKGROUND OF INVENTION

Reference is made hereinafter to the following documents:

-   1. www.colorado.edu/˜bart/book/solar.htm: Bart J. Van Zeghbroeck,    1997, Chapter 4.8 (Photodiodes and Solar Cells) and Chapter Section    2.2.5 (Temperature and stress dependence of the energy bandgap).-   2. J. G. Mavroides, J. A. Kafalas, and D. F. Kolesar,    “Photoelectrolysis of water in cells with SrTiO₃ anodes,” Applied    Physics Letters, Vol. 28, No. 5, 1 Mar. 1976, and references    therein.-   3. A. Fujishima and K. Honda, Nature, 238, 37 (1972)-   4. O. Khaselev and J. Turner, “A Monolithic    Photovoltaic-Photoelectrochemical Device for Hydrogen Production via    Water Splitting,” Science, Vol. 280, 17 April, 1998.-   5. P. J. Sebastian, M. E. Calixto, and R. N. Bhattacharya,    Electrodeposited CIS and CIGS thin film photocatalysts for hydrogen    production by photoelectrolysis.-   6. T. Gerfin, M. Graetzel and L. Walder, Progr. Inorg. Chem., 44,    345-393 (1997), Molecular and Supramolecular Surface Modification of    Nanocrystalline TiO₂ films: Charge-Separating and Charge-Injecting    Devices.-   7. Guerra, J. M., Storage Medium Having a Layer of Micro-Optical    Lenses Each Lens Generating an Evanescent Field, U.S. Pat. No.    5,910,940, Jun. 8, 1999.-   8. Guerra, J. M., Adsorption Solar Heating and Storage System, U.S.    Pat. No. 4,269,170, May 26, 1981.-   9. Guerra, J. M., Photon tunneling microscopy applications, MRS    Proceedings Volume 332, Determining Nanoscale Physical Properties of    Materials by Microscopy and Spectroscopy, M. Sarikaya, H. K.    Wickramasinghe and M. Isaacson, editors. Page 457, FIG. 8b shows    tensile stress fissures in diamond-like carbon coating on a silicon    substrate. FIG. 9a shows adhesion failure due to compressive    stresses in a magnesium fluoride thin film coating on an acrylic    substrate.-   10. Guerra, J. M., Storage Medium Having a Layer of Micro-Optical    Lenses Each Lens Generating an Evanescent Field (application title:    Optical Recording Systems and Media with Integral Near-Field    Optics), U.S. Pat. No. 5,910,940, Jun. 8, 1999. Assigned to Polaroid    Corp.-   11. Guerra, J. M. and D. Vezenov, Method of fabrication of    sub-micron spherical micro-lenses. Patent Applied For Apr. 12, 2001.-   12. Guerra, J. M. et al, “Embedded nano-optic media for near-field    high density optical data storage: modeling, fabrication, and    performance,” Proceedings, Optical Data Storage Conference, SPIE,    April, 2001.-   13. Guerra, J. M. et al, “Near-field optical recording without    low-flying heads,” ISOM Technical Digest, Taipei, 2001.-   14. Guerra, J. M. et al, “Near-field optical recording without    low-flying heads: Integral Near-Field Optical (INFO) Media,”    Japanese Journal of Applied Physics, scheduled publication March    2002-   15. J. M. Bennett et al, “Comparison of the properties of titanium    dioxide films prepared by various techniques,” Appl. Opt. 28,    3303-3317 (1989)-   16. H. T. Tien and A. L. Ottova, “Hydrogen generation from water    using semiconductor septum electrochemical photovoltaic (SC-SEP)    cells,” Current Topics in Biophysics 2000, 25(1), 39-60. Modeled on    nature's photosynthetic thylakoid membrane.-   17. D. Ginley et al, “Nanostructured interfaces in polymer based    solar cells,” National Renewable Energy Laboratory, SERF, Golden,    Colo. Deposition of MEH-PPV and cyano-PPV on nanostructured TiO₂ for    thin film photovoltaic cells.

This invention relates to photocatalytic surfaces used in the process ofphotoelectrolysis, photovoltaics, and photocatalysis, and morespecifically to induction and management of stress in a thin titaniafilm photocatalytic surface to match the band gap of the titania moreefficiently with the solar spectrum at the earth's surface forphotoelectrolysis, photovoltaics, and photocatalysis.

The ills of our carbon-based energy are well-known: pollution of landand oceans, air pollution, and the global warming that is likely causedby the latter. In addition, there is the growing dependence on foreignoil (presently at 46%, up from 27% during the Oil Embargo during theCarter administration) with the economic, political, and human coststhat result from that dependence. Hydrogen has been gradually emergingas the fuel of choice for the future and perhaps even the very nearfuture. Fuel cell technology has recently advanced exponentially, withplans for miniature fuel cells to replace batteries in the everpower-hungry personal digital devices, and for combustion engines forautomobiles in which hydrogen is the fuel. This last importantapplication has made great progress in that the hydrogen can now besafely and efficiently stored in a host of metal hydride basedmaterials, with the hydrogen being piped to or stored at local fillingstations, with the associated cost and danger. In another approach, thehydrogen is split at the engine from toxic hydrogen-bearing liquids suchas gasoline and alcohols.

Ultimately, for a hydrogen-based energy to be completely beneficial, onewould like to be able to split our most abundant resource, water, with arenewable energy source. Many have turned to solar cells to provide theelectricity for electrolysis of water as a way to provide a stable andefficient storage for solar energy, with the stored hydrogen (adsorbedin a metal hydride, Ovshinsky et al) later used to create electric powerin a fuel cell. However, the losses of the solar cell in convertingsunlight to electricity, combined with the losses in the electrolyticsplitting of water into hydrogen and oxygen, make for low efficiencyoverall. Further, the cost of the apparatus and lifetime of thecomponents make the economic viability dim at this time.

A promising path and highly sought-after goal is to use sunlightdirectly to split water. The free energy required for decomposing waterinto gaseous H₂ and O₂ is just 1.23 eV, so this seems possible giventhat the peak of the solar spectrum is about 2.4 eV (ref. Mavroides).However, the threshold energy for this reaction is 6.5 eV, so directphotodissociation is not possible. However, Honda and Fujishima (Nature238, 37 (1972)) showed that the threshold energy required can be greatlyreduced by introducing a photocatalytic semiconductor surface, such astitania. Immersing single crystal titania (n-type) and Pt electrodes inan aqueous electrolyte, connected externally to form an electrolyticcell, they observed development of gaseous oxygen at the titaniaelectrode and gaseous hydrogen at the Pt electrode when the cell wasilluminated. (In other photoelectrolytic cells, hydrogen collects at thesemiconductor cathode and oxygen collects at the conducting anode, witha membrane preventing their recombining.) However, while they succeededin activating titania as a photocatalyst, they required artificiallight, such as a xenon lamp, with a photon energy of greater than 3.0eV, the energy gap of titania. Even so, their energy conversionefficiencies were low. Further, such light is in the ultraviolet part ofthe spectrum, and very little is present in sunlight at the surface ofthe earth (sunlight integrated over the 3 eV to 4 eV range is only 4 mWper square cm, compared to the 100 mW per square cm total in visiblesunlight), so that titania photoelectrolysis with sunlight has less than1% efficiency, and the photoelectrolysis quantum efficiency, independentof the solar spectrum, is only 1-2% unless a bias voltage is applied.For photoelectrolysis, as it is known, to spontaneously occur insunlight, and with a practical efficiency, therefore requires thesemiconductor to have a bandgap of about 1.7 electron volts (eV) inorder to (1) have the energy required to split the water into hydrogenand oxygen gases, and (2) absorb at the peak of the solar spectrum forhighest efficiency.

Following this work, others (Turner and Warren) have investigatedsemiconductor alloys or compounds with lower bandgaps. For example,p-type GaInP₂ has a bandgap of 1.8 to 1.9 eV, which would workadequately in sunlight to produce a photocurrent that can be used tobreak down water into hydrogen and oxygen. However, they found thatsurface treatments in the form of metallated porphyrins and transitionmetals, such as compounds of ruthenium, were necessary to suppress thebandedge migration and allow bandedge overlap to occur. Without thistreatment, hydrogen and oxygen cannot be produced because the conductionband and the Fermi level of the semiconductor do not overlap the redoxpotentials of water, i.e. when light shines on the semiconductor,electrons build up on the surface, shifting the bandedges and Fermilevel further away from the overlap of the water redox potentials. Thelong term surface stability of these surface treatments are not known.

Other attempts at photoelectrolytic cells with lower bandgapsemiconductors typically (1) are corrosive in water, and (2) require abias voltage, supplied by a conventional power source or by aphotovoltaic cell or photodiode. The corrosion problem has been reducedby using platinum as the anode, and/or by combining differentsemiconductors. This again reduces economic viability.

The titania electrode in the Honda/Fujishima cell has the importantadvantage that it does not undergo anodic dissociation in water, andtitania is much less expensive than other semiconductors. Mavroides,Kafalas, and Kolesar demonstrated somewhat higher efficiency titaniacells using n-type SrTiO₃ for its smaller electron affinity, afterhaving confirmed the Honda/Fujishima results with titania in earlierwork. They achieved 10% maximum quantum efficiency, an order ofmagnitude higher than for titania, but with light with energy hυ (whereh is Planck's constant and υ is the light frequency) at 3.8 eV, comparedto 3.0 eV required for titania. They believed this increase inefficiency was the result of band bending at the anode surface that isabout 0.2 eV larger than for titania, resulting from the smallerelectron affinity of SrTiO₃. In their energy-level model forphotoelectrolysis, the semiconductor serves as only the means forgenerating the necessary holes and electrons, without itself reactingchemically. In their model, the low quantum efficiency of titania is notdue to inefficient carrier transfer, as others had shown that this wasclose to 100% with platinized −Pt cathodes and illuminated titaniaanodes, but rather to insufficient band-bending at the titania surfaceto cause efficient separation of the electron-hole pairs. The completeprocess, according to their model as in Ref 2, (which is in substantialagreement with models of other researchers), is that photoelectrolysisoccurs because electron-hole pairs generated at the semiconductorsurface upon absorption of illumination with the required photon energyare separated by the electric field of the barrier, in the form of theenergy-band bending at the surface, preventing recombination. Theelectrons move into the bulk of the anode and then through the externalcircuit to the cathode. There, they are transferred to the H₂O/H₂ levelof the electrolyte and hydrogen gas is released:

2e ⁻+2H₂O→H₂+2OH⁻  (1)

Oxygen is produced at the same time as holes are transferred from theanode surface to the OH⁻/O₂ level of the electrolyte, as:

2p++2OH⁻→½O2+H2O  (2)

In other work that is farther a-field from this application, Graetzelinvented a titania solar photovoltaic cell in which the functions ofabsorption of light and the separation of the electric charges(“electrons” and “holes”) are not both performed by the semiconductor(titania in this case). Instead, the light absorption is performed by adye monolayer that is adsorbed onto titania particles, in one case, andonto titania nano-crystals, in another case. In this way he avoids theproblem of titania's 3.0 eV bandgap. This technology is now beingcommercialized by, for example, Sustainable Technologies International.Others have followed his lead and replaced the dye absorber with quantumdot particles attached to the titania particles, where the quantum dotsperform the light absorption (QD Photovoltaics, The University ofQueensland). In all of this work, however, there is no attempt to alterthe bandgap of the titania. Also, the titania layer is required to bemicrons thick, and is applied as a sol-gel. Such a process requiressolvents and temperatures incompatible with polymer substrates. Further,an electrolyte is required to fill the porous gaps in the titania matrixand complete the cell. This electrolyte is non-aqueous and somewhatvolatile, so packaging, cell lifetime, and effect on the environmentremain problematic. Efficiencies are reported to be around only 5% atthis point. Most importantly, such a device provides no direct access tothe titania photocatalytic surface, and so cannot be used for hydrogenproduction, detoxification, or disinfection.

Still further a-field is work by researchers at Oxford's Physics andChemistry Departments, who are devising “inverted” photonic bandgap(PBG) crystals comprising polycrystalline titanium dioxides (titania),while earlier researchers achieved the same with self-assembled titanianano-spheres. Here, the bandgap is determined by the relative indices ofrefraction of the titania spheres and the empty or lower index mediaaround and in between the spheres, the size of the spheres, and theirgeometrical arrangement. Again, there is no attempt to alter the bandgapof the titania spheres themselves, and the application is for directing,absorbing, and otherwise controlling light of a certain wavelength. Thetitania is used for its high refractive index of 2.4 to 2.6, whichprovides the desired index ratio of greater than 2 to if the immersionmedium is air with in index of unity.

So, titania has also been shown to have use in photovoltaic devices. Andin addition to photoelectrolysis for hydrogen production, titania'sphotocatalytic properties have been shown to have beneficial applicationto disinfection by killing biological organisms, and detoxification bybreaking down toxic chemicals. It will be seen that the inventiondisclosed herein, by enabling titania to function well in visible light,such as sunlight, also applies to photovoltaics, disinfection, anddetoxification.

In all of the above work, titania is either in the form of a slab cutfrom a crystal, and can be either of the most common polymorphs rutileor anatase, or is a thick film resulting from a sol gel process, or elseare small particles of crystalline titania either in suspension orhot-pressed into a solid. No one is using, to our knowledge, titania inthe form of a thin film deposited in a vacuum coating process.

SUMMARY OF THE INVENTION

One would like a semiconductor photocatalyst with a bandgap that isbetter matched to the solar spectrum and/or artificial illumination forhigher efficiency or even to work at all. In this invention, the bandgapof the known chemically-inert photocatalyst titania (TiO₂) is shiftedand broadened to be active at wavelengths more prevalent in sunlight andartificial light by inducing and managing sufficiently high stress intitania by vacuum coating a thin film of titania onto a substrate,preferably of a different Young's modulus, with bending undulations onthe surface of a spatial radius similar to the film thickness. Theundulated coating also serves to self-focus and concentrate the incidentlight required for the process, increase photocatalytic surface area,and prevent delamination of the film from the substrate. The electricalactivity so induced in the band-shifted titania subsequently by visiblelight is applied to photoelectrolysis (hydrogen production from waterand light), photovoltaics (electrical power from sunlight),photocatalytic disinfection and detoxification, point-of-usephotoelectrolysis for use in internal combustion engines, for example,and stress-induced tunable bandgap components for communications. Inaddition, the same stress-induced thin film bandgap shifting works withother semiconductors such as amorphous silicon, and with similarbenefits.

Accordingly, this invention provides for shifting, lowering, or reducingthe size of, the optical bandgap of a semiconductor into opticalwavelengths predominant in the illuminant by stressing thesemiconductor, where the semiconductor is a thin film, and/or where thestress is caused by conditions under which the thin film is coated,and/or where the stress is caused by the shape of the substrate on anano, micro, or macro scale, and/or where the stress is caused by themechanical, chemical, and thermal properties of the substrate.

In accordance with further features of the invention, the bandgap isshifted into longer wavelengths by heating.

In accordance with further features of the invention, the semiconductoris titania.

In accordance with further features of the invention, the bandgap isshifted into wavelengths that are abundant in the solar spectrum.

In accordance with further features of the invention, the semiconductoris a photocatalyst.

In accordance with further features of the invention, thestress-inducing template profiles also provide a mechanical lock to thecoating so that the stress can exist without causing delamination of thecoating from the substrate.

In accordance with further features of the invention, thestress-inducing template profiles create additional surface area withoutincreasing the width or length of the surface, for additional efficiencyin photocatalytic action.

In accordance with further features of the invention, the photocatalystis used to split an aqueous solution into hydrogen gas and oxygen gaswhen irradiated.

In accordance with further features of the invention, the illuminationis from the sun.

In accordance with further features of the invention, the illuminationis from artificial light.

In accordance with further features of the invention, the illuminationis further concentrated by the stress-inducing template shape byself-focusing.

In accordance with further features of the invention, the concentrationof the illumination is largely independent of incident illuminationangle, thereby reducing or eliminating the need to track the sun in thesky.

In accordance with further features of the invention, thestress-inducing profiles in the substrate may be one-dimensional, suchas cylinders, or two-dimensional, such as spheres.

In accordance with further features of the invention, the pitch of thestress-inducing profiles may be chosen relative to the desiredillumination wavelength such that absorption is increased and reflectionis decreased as in a photonic bandgap crystal.

In accordance with further features of the invention, the thickness ofthe titania layer is chosen to be ¼ of the wavelength of the desiredillumination, thereby acting as an anti-reflection filter and increasingabsorption and decreasing reflection.

In accordance with further features of the invention, the substratesurface profiles are chosen to be a certain shape, depth, and radius sothat the titania film grows as lenses over the profiles.

In accordance with further features of the invention, the thickness ofthe titania is chosen so that the focal plane of said lenses is coplanarwith the distal surface of the titania layer.

In accordance with further features of the invention, the additionaleffective surface created by the substrate stress-inducing profilesfacilitates and improves heat dissipation.

In accordance with further features of the invention, the semiconductoris vacuum coated onto or into the shaped substrate.

In accordance with further features of the invention, the semiconductoris applied as a sol gel.

In accordance with further features of the invention, the semiconductoris applied with chemical vapor deposition.

In accordance with further features of the invention, the semiconductoris a contiguous film.

In accordance with further features of the invention, the semiconductoris a matrix of particles such as spheres.

In accordance with further features of the invention, the substrate canbe polymer, glass, silicon, stainless steel, copper, aluminum, orsubstrate material.

In accordance with further features of the invention, the photocatalystis used to detoxify a medium in contact with it.

In accordance with further features of the invention, the photocatalystis used to disinfect a medium or biological agent in contact or proximalwith it.

In accordance with further features of the invention, the substrate istransparent.

In accordance with further features of the invention, the substrate isreflective.

In accordance with further features of the invention, the substrate canbe flexible.

In accordance with further features of the invention, the substrate andtitania coating are compatible with a roll-to-roll web manufacturingprocess.

In accordance with further features of the invention, the substrateprofiles are embossed into the substrate with a stamper from a master.

In accordance with further features of the invention, the substrateprofiles are molded into the substrate with a stamper from a master.

In accordance with further features of the invention, the substrateprofiles are caused by reticulation in the substrate or in a layerapplied to the substrate.

In accordance with further features of the invention, the titania-coatedsubstrates can be stacked in layers to increase efficiency for a givenarea.

In accordance with further features of the invention, saidtitania-coated stacked substrates may be pre-coated on the opposite sidewith a transparent conducting electrode such as indium tin oxide (ITO).

In accordance with further features of the invention, the titania-coatedsubstrates are edge-illuminated.

In accordance with further features of the invention, the semiconductoris strontium titanate (SrTiO₃), amorphous silicon, or othersemiconductor.

In accordance with further features of the invention, the titania-coatedsubstrate(s) function as the anode in a photoelectrolytic cell, whichfurther comprises some or all of the following: a housing, an aqueouselectrolyte, a separation membrane, a cathode, and a bias source.

In accordance with further features of the invention, where thephotoelectrolysis stores solar energy chemically in the form ofhydrogen, and may be further combined with a metal hydride or otheradsorber.

In accordance with further features of the invention, the nano-lensesimprove performance in low light level and diffuse light applications.

In accordance with further features of the invention, the applicationsare for both terrestrial and space environments.

In accordance with further features of the invention, the illuminant isa laser diode or laser.

In accordance with further features of the invention, the illuminant isa spark between electrodes.

In accordance with further features of the invention, the illuminant isa flashlamp.

In accordance with further features of the invention, the hydrogen isproduced at point of use by artificial illumination.

In accordance with further features of the invention, the substrateshape is used to increase or otherwise control the stress in the titaniafilm.

In accordance with further features of the invention, the shape can beconcave or convex or a mix of both, and the scale of the radius ofcurvature can be from nanometers to meters.

In accordance with further features of the invention, the substrate is apiezo device such that the amount of stress induced in the titania film,and therefore the bandgap, is tunable over a range, for use inphotonics.

In accordance with further features of the invention, the substrate istemperature controlled, such that by changing the temperature thesubstrate contracts or expands and causes a tunable bandgap shift in thetitania or other photocatalyst layer.

In accordance with further features of the invention, the substrate is avery small particle or sphere, typically several microns in diameter butas small as tens of nanometers in diameter, and the material is apolymer, glass, metal, or other material, and is coated with titania orother suitable semiconductor.

In accordance with further features of the invention, said sphere is oneof many applied to a surface or surfaces, or are in suspension in afluid, and can be applied by spraying, painting, or inkjet deposition.

In accordance with further features of the invention, the substrate is asmall diameter polymer or other fiber, and the titania-coated fiber iswoven into fabrics for protective clothing, or into mesh filters forwater or air filtration, or into paper for envelopes that are readilyanti-biotic when illuminated with daylight or artificial light.

In accordance with further features of the invention, the application isphotovoltaic, and the stress is enabling (titania) or improving(amorphous silicon).

In accordance with further features of the invention, the application isphotoelectrolysis.

In accordance with further features of the invention, the application isdetoxification.

In accordance with further features of the invention, the application isdisinfection.

In accordance with further features of the invention, the application ispoint-of-use photoelectrolysis.

In accordance with further features of the invention, the application iscontinual tuning of stress and bandgap properties for telecommunicationapplication.

In accordance with further features of the invention, the application isto alter and improve magnetic properties of thin films applied to harddrive disks for data storage.

In accordance with further features of the invention, the application isto provide a corrugated substrate to which a desired titania or otherthin film will adhere under stress but will not cause scatter ordiffraction due to its sub-wavelength spatial period.

In accordance with further features of the invention, the sinusoidalinterface at the high index thin film and low index substrate or lowindex air or water respectively further cause an effective index thatvaries gradually from one index to the other, with gradient indexbenefits of improved transmission and reduced reflection.

In accordance with further features of the invention, the photocatalystis a thin film, thereby reducing the probability of recombination of thehole and electron pairs that occurs in bulk semiconductors in theabsence of an anode (or cathode) and electrolyte.

In accordance with further features of the invention, the titaniacoating is evaporated from a titania target, a titanium target withoxygen bled into chamber, or a Ti_(x)O_(y) target such as Ti₂O₃.

In accordance with further features of the invention, the titaniacoating may comprise rutile and/or anatase and/or other polymorphs, aswell as amorphous titania.

In accordance with further features of the invention, additional thinfilms may be applied between the titania and the substrate in order topromote adhesion or to further modify the stress in the titania.

In accordance with further features of the invention, the combustionproduct is clean desalinated water, so that the photocatalytic deviceprovides desalination and purification of water.

In accordance with further features of the invention, the point-of-usephotocatalyst device is used in a hydrogen-based internal combustionengine.

Other features of the invention will be readily apparent when thefollowing detailed description is read in connection with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The structure and operation of the invention, together with objects andadvantages thereof, may best be understood by reading the detaileddescription to follow in connection with the drawings in which uniquereference numerals have been used throughout for each part and wherein:

FIG. 1 is a scanning electron photomicrograph of a section through of astressed titania fill of the present invention formed by growing titaniaon to a polycarbonate template having a sinusoidal surface.

FIG. 2A is a scanning electron photomicrograph, similar to that of FIG.1, but showing titania grown on a polycarbonate template having acylindrical surface.

FIG. 2B is a scanning electron photomicrograph, similar to that of FIG.2B, but in which the peak to valley depth of the cylinders is greaterthan in FIG. 2A;

FIG. 3A is a schematic cross-section through a stressed film formed on aundulating substrate and showing the stresses developed.

FIGS. 3B and 3C are schematic cross-sections, similar to that of FIG.3A, showing respectively compressive and tensile stresses in layerformed on a planar substrate.

FIG. 3D is a schematic cross-section, similar to that of FIG. 3A,showing the stresses in a bent planar substrate.

FIG. 4 is a schematic cross-section through a photoelectrolysisapparatus of the present invention.

FIG. 5 is a schematic side elevation, partially in cross-section, of asecond photoelectrolyis apparatus (cell) of the present invention.

FIG. 6 is a schematic side elevation of one method for manufacturing astressed electrode of the present invention.

FIG. 7 is a schematic cross-section through a multi-layerphotoelectrolysis apparatus of the present invention.

FIG. 8 shows the multi-layer photoelectrolysis apparatus of FIG. 7 inoperation, and the optical means used for illuminating the apparatus.

FIG. 9 is a schematic cross-section through a photovoltaic cell of thepresent invention.

FIG. 10 is a schematic cross-section through a photocatalytic apparatusof the present invention being used for detoxification and/ordisinfection.

FIG. 11A is a view, partly in section, of an electrode of the presentinvention coated on a polymer fiber.

FIG. 11B is a view, partly in section, of an electrode of the presentinvention coated on a polymer sphere.

FIG. 12 shows a fabric and a filter mesh assembled from the fibers andspheres shown in FIGS. 11A and 11B respectively.

FIG. 13 shows is a schematic cross-section through a point-of-usephotoelectrolysis apparatus of the present invention used to feedhydrogen and oxygen gas to an internal combustion engine.

FIG. 14 is a schematic cross-section showing how a photodiode and aphotocatalytic surface may be combined in a photoelectrolysis apparatusof the present invention.

FIG. 15 shows schematic cross-sections through two stressed electrodesof the present invention formed on piezoelectric crystals as substrates.

DETAILED DESCRIPTION

It is known that the bandgap of a semiconductor can be altered by (1)doping, (2) adding stress, and (3) adding heat. Herein, I disclosemaking use of the stress that is inherent in thin films, andspecifically the tensile stress, to shift the bandgap of a semiconductorfurther into the visible spectrum. Bandgap-shift from local heating fromself-focusing of the illuminant in the film is also disclosed ascontributing to the effect, but this appears to be a secondary effect inthis invention. For example, the energy bandgap of GaAs, or galliumarsenide, requires a 900° C. change in temperature to drop only 0.4 eV,from 1.5 eV at 100° C. down to 1.1 eV at 1000° C. On the other hand,significantly higher magnitude changes in stress can be achieved in thisinvention, and so stress is the predominant factor in the lowering ofthe bandgap energy.

When tensile stress is applied to or caused in a semiconductor, theinter-atomic spacing increases directly. An increased inter-atomicspacing decreases the potential seen by the electrons in the material,which in turn reduces the size of the energy bandgap. The same effectoccurs with increased temperature, because the amplitude of the atomicvibrations increases with the increased thermal energy, thereby causingincreased inter-atomic spacing. The main feature of this invention,accordingly, is that the stress is carefully controlled to achieve thedesired bandgap shift, and further managed to prevent delamination, byintroducing periodic three-dimensional nano-scale surface features intoor onto the substrate. These features act as a template such that thefilm that is grown onto the template takes on a similar shape. FIG. 1.is a scanning electron micrograph of a titania film 1 grown onto apolycarbonate template 3 comprising a close-packed three dimensionalsinusoid surface 2, much like an egg carton, with a spatial period of300 nanometers (nm) or 0.3 microns. FIG. 2A is a scanning electronmicrograph of a cross-section of another polycarbonate template 4, inwhich the surface geometry comprises cylinders 5 with a spatial periodof 300 nm, which result in stressed titania layer 6, which is immersedin air 7 in this figure. This is an example of a conformal coating, inwhich the undulations in the titania layer have the same shape andprofile as in the substrate. FIG. 2B is a scanning electron micrographof a cross-section of yet another polycarbonate template 8, in which thesurface geometry 9 again comprises cylinders with a spatial period of300 nm, but the peak to valley depth is larger than in FIG. 2A. Thisresults in the titania coating 10 having a final shape that is morecylindrical than sinusoidal, an example of a non-conformal coating,which in turn changes both the stress characteristics in the film aswell as the optical self-focusing characteristic. The titania coating 10is facing a medium 11 that is air in this image, but forphotoelectrolysis this would be an aqueous or hydrogen-bearing liquid.

FIG. 3A. is a cross-sectional drawing of substrate 17′ that is shown tohave undulations 17 on the surface that transfer to the titania coating16 applied by a vacuum technique. The titania coating has a filmthickness 13 of about 200 nm, although it can be thinner or thickerdepending on the coating conditions and the geometry of the substrate.As was seen in FIG. 2., the titania coating undulations can be madeconformal, i.e. they follow the curves in the substrate exactly, or theycan be made more like adjacent hemi-cylinders, with sharp cusps inbetween, depending on the coating film growth conditions and target tosample geometry, and the substrate undulation geometry: peak to valleyheight 15, radius of curvature 14, and pitch 12. For this figure thecoating is shown to be conformal. For polycarbonate as the substratematerial, thermal contraction of the polycarbonate 17′ after coating islarger than for the titania thin film. The result is that very highstresses are formed in the titania layer: tensile stress 18 in the apexof the undulations, and compressive stress 19 in the troughs. Suchcompressive and tensile stresses are present in thin film coatings onplanar substrates as well, depending on whether the substrate grows orshrinks, respectively, relative to the film after coating, and suchsurfaces are within the scope of this invention when applied tostress-induced bandgap shifting, particularly of titania. FIG. 3B is across-section of such a planar substrate 21 and titania coating 20, inwhich the stress 23 is compressive, and FIG. 3C. is a cross section ofplanar substrate 26 and planar coating 25 which is in tensile stress 24.FIG. 3D is a cross section of a planar film 27 that is bent by upwardforce 28 and downward forces 29 and 30, which results in tensile stress31 and compressive stress 32. These bending stresses are the basis forthe stress induction in this invention. However, the undulations in thepreferred embodiment create much higher tensile stresses because of theintroduction of very small localized bending radii 14 in FIG. 3A. Inaddition, the undulations provide a mechanical lock between thesubstrate and the coating, which allows the high stresses to existwithout delamination. Other benefits of these undulations will be seenlater in the description.

The resulting bandgap-shifted semiconductor, titania in the preferredembodiment, is then applied to photoelectrolysis for hydrogenproduction, photovoltaics for electricity production, and photocatalysisfor detoxification and disinfection. Other semiconductors, including butnot limited to strontium titanate, amorphous silicon, hydrogenatedamorphous silicon, nitrogenated amorphous silicon, polycrystallinesilicon, and germanium, and combinations thereof, will also exhibit ashift in their respective band-gaps toward a more favorable part of thesolar spectrum with this applied stress.

Thin films, whether for optical, magnetic, semiconductor, or otherapplication, and whether of dielectric, metallic, semiconductor, orother material, are typically inherently stressed as applied. Stress inthin films leads to delamination, also known as adhesion failure, andcan also change the optical, magnetic, or electronic properties of thefilm. Therefore, stress in thin films is normally thought of assomething to be managed or tolerated rather than as something useful.Stress is controlled by, and in this disclosure also meaning induced by,the following factors (this list not intended to be limiting orall-inclusive): (1) film thickness (2) rate of application (film growth)measured in angstroms/second (3) mean free path and vacuum level (4)e-beam energy (5) match of film and substrate mechanical and thermalcoefficients (6) shape of the substrate on both a nanometer and macrolevel (7) target material from which the film is evaporated, (8)distance of the substrate to the source (tooling factor), (9) thepresence or absence of mitigating layers, and (10) implanting ionsduring or after deposition to change the material and stress. Stress inthe film can be made to be either tensile or compressive, and is inducedalso by bending. Stress in the film can also be photo-induced,especially if the film is self-formed into internally or self-focusingoptics.

The films described here are contiguous thin films, rather thanparticles. However, it is known in the art than on a nano and microscale all thin films exhibit some crystalline structure, particulateaggregation, and porosity. Also, small polymer or other particles can becoated with titania to form the stress shifting on a particle level, andthese particles can be applied through, for example, a spray, or in asuspension applied by inkjet, or simply painting them on, suspended in abinder. Further, a titania coating can be applied to fibers, especiallypolymer fibers, to achieve the same stress-induced benefits. Thesefibers can then be woven into protective fabrics for garments, into airfilters, or into paper for antibiotic envelopes.

Titania films can be formed with chemical vapor deposition, sol-gel, orvacuum coating, for example. With chemical vapor deposition, thematerial is deposited as an ash, which then coalesces into a contiguousfilm upon application of heat from, typically, a gas flame. Sol gelcoatings have the titania particles in a solvent that is spun coated ordipped or otherwise applied to a substrate. If the solvent is drivenoff, the film that remains is a porous matrix of titania particles. Thisis done typically for the dye-adsorbed solar application of titania. Thefilm can be several microns thick, and the dye within the porouscavities increases the amount of surface area for interaction. If thisfilm is baked, the titania particles will coalesce into a reasonablycontiguous film.

While it is possible to induce the required stress with the abovecoating approaches, thin film vacuum deposition is preferred forinducing the highest stress and with the lowest amount of material.Typical vacuum deposition methods include sputtering, electron-beam, andion deposition, for example. My earlier work with these structures,which at this size are nano-optics, for the application of increasingoptical data storage density, has shown that they indeed focus incidentlight and increase the power density at their focal plane (rigorouselectromagnetic modeling, thermal finite element modeling, and empiricalresults with phase change materials placed at the focus plane allcorroborate this). Further, their sinusoidal to cylindrical shapenaturally gives rise to significant tensile stress. The inventiondisclosed herein is that one or both of these factors are causing thebandgap to drop to a level at which spontaneous photoelectrolysis ofwater can occur in a spectral region that is abundant in sunlight.Stress in thin films can be controlled by a combination of filmthickness, substrate-to-film match or mismatch of thermal and mechanicalcoefficients, micro or nano-scale shape, and by the addition of otherthin films.

While there is certainly a benefit to having the stress-inducing shapeperform also as a light concentrator, it will be clear that otherstress-inducing template profiles may be used, even if they do not alsoact as light concentrators.

Stress in thin films can be as low as 2 MPa, but is more typically up to100 MPa, and can reach into the GPa's depending on the coatingconditions, the thin film or thin film stack, and the substrate (the SIstress unit is the Megapascal, or MPa, and is equivalent to the Englishstress unit of pounds per square inch, or PSI; GPa is Gigapascal, or onebillion pascals). The stresses can be so high that a thin dielectricfilm only a couple of hundred nanometers thick can cause a substrate asthick as a millimeter to be noticeably distorted by bending, and in factthis distortion is used to monitor stress optically by observingdeflection of light from such a bending substrate during coating.

The stress in the thin film σ can be expressed to first order as theintrinsic film stress σ_(i) caused by the coating conditions plus stressσ_(e) from an external bending force F (in Newtons N):

σ=σ_(i)+σ_(e)  (3)

where it is assumed that the Young's moduli of film and substrate areequal. However, additional stress can be induced in the thin film whenYoung's moduli of film and substrate are decidedly unequal. Moreover, ifthe substrate/film interface is undulating on a spatial scale of thesame order of magnitude to the thickness of the coating, such unequalmoduli result in significant bending forces on the film. The relation ofthe external stress σ_(e) to the bending force F is:

σ_(e) /F=12 MPa/N  (4)

It can be seen from equation (4) therefore, that even small externalforces are leveraged into large stresses in the film. A film of titaniaonly 125 nm thick, deposited on a thick polycarbonate substrate having asurface embossed or otherwise formed into a sinusoidal, hemispherical,or hemi-cylindrical geometry with a spatial period of 370 nm, canexperience stress in the GPa range and higher, sufficient tosignificantly alter the bandgap. Such stresses in planar thin films cancause the films to crack and even delaminate from the substrate, wherein the compressive stress case the film behaves much like the earth'scrust in plate tectonics, and literally explodes away from the surface,leaving a gaping crack (FIG. 9a in Guerra, “Photon Tunneling MicroscopyApplications”). In the tensile stress case, the film pulls away fromitself, leaving a crack in the coating that scatters light (FIG. 8b inGuerra, “Photon Tunneling Microscopy Applications”). Scientists in thethin film coating world usually strive to reduce stress in thin films,accordingly. However, it is recognized that some level of stress willalways be present in a thin film, and so when stresses causedelamination, they refer to this as adhesion failure, in recognitionthat controlling and increasing adhesion between the layer and thesubstrate will allow the layer to remain intact in the presence ofmodest internal stress if the adhesion is high enough. In thisinvention, the corrugated substrate not only causes the film to be in ahighly stressed state, but also locks the film to the substrate andprevents delamination, even though in a highly stressed state.

As was seen in FIG. 3., titania at the apex of the sinusoidal geometryexperiences tensile stress, while the titania in the trough of thesinusoid experiences compressive stress. For the applications ofphotoelectrolysis, detoxification, and disinfection, this means that thedesired photocatalytic activity is induced in the titania at the part ofthe surface (apex of the sinusoid) closest to the object ofphotocatalysis. Because the stress varies continuously from tensile tocompressive, the bandgap is not only shifted but broadened as well.Further, more of the film is in a beneficially stressed state than wouldbe the case for a bulk form of the titania, where the stress would belargely near the surface and a comparatively much smaller percentage ofthe bulk volume of the semiconductor.

The description of an illustrating experiment and results follows, andis shown in FIG. 4.

It is known that the bandgap of a semiconductor can be markedlydecreased by (1) applying tensile stress and/or (2) elevating itstemperature. In fact, in semiconductor devices this is known as “packageshift”, in which for example a bandgap reference is shifted in voltageafter packaging in plastic, from the package-induced stresses resultingfrom the thermal coefficient mismatch between the encapsulating plasticand the silicon device. Unlike that example, however, in which theeffort and desire is to reduce the stress and resulting bandgap shift,herein the shift is a beneficial effect that one would like to amplifyand control. So, one would like a surface coated with TiO₂ that appliestensile stress to the TiO₂ layer (and perhaps elevates the temperatureas well). For this photoelectrolysis application, TiO₂ 36 a was coatedonto a polycarbonate surface 34 into which had been formed, by moldingreplication from a nickel stamper, undulations 35 in the form ofsinusoidal to cylindrical profiles. The TiO₂ grows on this templateshape during the vacuum coating process to form lenses. For thisexperiment the lenses happened to be cylinders arranged in a continuousspiral track, with the pitch of the cylinders, and therefore theirwidth, being 370 nm. The polycarbonate substrate is 0.6 millimetersthick, and the titania film is 210 nanometers thick.

The experimental apparatus comprised a Nikon optical microscope with atungsten-halogen 50 W light source. A 40×0.6 numerical aperture (N.A.)objective focused the light 33 down to the polycarbonate substrate 34,with the planar surface facing the microscope. The corrugated surface(370 nm pitch) coated with the 210 nm thick TiO₂ faced a first-coatedaluminum 36 c on glass 36 d mirror. Tap water 36 b was dripped into theinterface, forming and aluminum/water/TiO₂ sandwich. Focus was adjustedto cause the brightest back-reflection, and then the experiment wasvisually checked periodically through the microscope.

After an elapsed time of 10 minutes, bubbles were observed inside butnear the edge of the illuminated field. These bubbles rapidly increasedin number over the next few minutes until they began to merge.Eventually, the merged bubbles created a zigzag geometry similar to thatobserved when free surface coatings de-wet from the surface. Theorientation of the zigzag stripes were observed to be perpendicular tothe cylinder axis, and this repeated in subsequent experiments. Thiszigzag pattern is also consistent with modeled bandgap change instrained semiconductors (Yang).

At lower power, Newton fringes were observed on a larger scalesurrounding the zigzag pattern, which was limited to the field of view.These fringes indicated a convex bulging distortion of the sandwich,caused by gas pressure.

With a green filter (540 nm) in place, the experiment did not repeat,thereby placing an upper limit on the degree of bandgap shifting in thiscase. The lower limit was determined by measuring the spectrum of theillumination at the focus of the microscope with an Ocean Opticsspectrophotometer, which showed no significant illumination below 490nm, and therefore one would not expect any photocatalytic action to beobserved in the titania, which requires illumination of about 390 nmwavelength because of its 3.0 eV bandgap.

In the flat TiO₂ coated areas of the same disc (i.e., where there are nocorrugations), nothing happened even after hours of exposure. Similarly,nothing happened with TiO₂ coated glass witness samples. This indicatesthat the corrugated surface profile is necessary, whether for thetensile stress or the optical temperature elevation it causes, or both.

Other lensed surfaces were tried. GaP and ZnS/SiO₂ showed no activityafter hours of exposure, indicating the semiconductor bandgap propertyof TiO₂ was required.

Other thickness TiO₂ coatings showed various results. At 190 nm, noactivity was observed. At 230 nm, activity was observed but took longer.This is consistent in that the stress in an optical thin film isdependent on, among other parameters, the film thickness. However, thismay indicate that the optical focusing is also important, along with thetemperature elevation associated with optical focus.

That the activity was restricted to the area inside the field of viewindicates that this is in fact a light-driven process.

The spectral output at the focus of the microscope is similar tosunlight, with no significant radiation below 490 nm wavelength. Thepower output at the focus, measured with a Newport photodiode with peaksensitivity at 520 nm, was 0.1 Suns.

The same sandwich was placed in sunlight, with a mask covering aportion. Once again, bubbles were observed even by eye and subsequentlyunder the microscope, and the bubbles stopped at the edge of the mask.

It should be noted that no attempt was made to make the TiO₂ aconductive n-type semiconductor, as was done in earlier studies, byheating to drive off oxygen (although, such films on planar glass,typified by their blue color, did not work here). Also, the water wasjust tap water, and not intentionally an electrolyte such as H₂SO₄ orNaOH. Further, no attempt was made to contact the anode to the cathodeto complete the cell. There may also have been an aluminum oxideovercoat on the aluminum mirror. Any dissolved oxygen in the water wasnot purged.

In the prior art discussed in the Background, when titania was used asthe photocatalyst, it was typically in the rutile form, and n-type, andwas a wafer cut from a rutile crystal. Others have used hot pressedtitania in a polycrystalline form, and others have used the anataseform, reporting a slightly better efficiency. No one is disclosing theuse of titania in the form of a vacuum deposited thin film, and no oneis disclosing thin films of titania deposited onto plastic substrateswith or without corrugations on the surface. In such a vacuum depositedthin film of titania as is being disclosed herein, the film can haveseveral material states: polycrystalline, amorphous, anatase polymorph,and rutile polymorph. How many and what kind of states exist and coexistin the film, and in fact even the stoichiometry, are determined by howthe film is deposited (e.g. e-beam), what substrate it is depositedonto, and what conditions were used in the coating process (substratetemperature, deposition rate, pressure, and starting target, forexample). These same conditions also control the level of stress in thefilm. For example, titania films deposited with e-beam evaporation aretypically amorphous, with higher refractive index n than with titaniafilms deposited by, say, ion assisted deposition. On the other hand,energetic ion- and plasma-based deposition produces denser titania filmsthat are also less rough than those deposited with e-beam. It is furtherknown that substrate temperatures above 380° C. result inpolycrystalline titania films of primarily the rutile phase, whilesubstrate temperatures of between 310° C. and 380° C. producepolycrystalline titania with both anatase and rutile phases. Titania canbe formed with a TiO or even Ti target and oxygen bleed-in during thedeposition, and this reactive evaporation results in predominantlyrutile titania, while starting with a Ti₃O₅ target results in anatasetitania. Other features of the titania film, such as density, roughness,resistance to water adsorption, and stress are also highly dependent onthe starting target material. For example, the Ti₃O₅ target is chosenbecause films made from it are lower stress, which is not a desirablefeature in this application. Optical absorption is another propertycontrolled by the starting material, and is reduced by a factor of 10with TiO as the target material, and by a factor of 100 with Ti₂O₃ orTi₃O₅ as the target. Of course, the science of coating, and inparticular the coating of titania films, is very complex and notcompletely predictable, and is largely outside the scope of thisapplication. Nevertheless, it should be recognized that this inventionincludes titania films formed by a number of different coatingtechniques, coating parameters, and starting materials. The discussionincluded here is only to indicate some of the controls that areavailable and possible to form titania with a specified material stateor states.

FIG. 5 is a cross-sectional drawing of a complete photoelectrolysis cellemploying stress-induced band-gap shifted titania as one of theelectrodes. Light 37 illuminates the polycarbonate substrate 38 thatalso comprises one side of the cell. The polycarbonate has a distalsurface 40 that has been embossed with undulations as have beendescribed in this specification, and coated with titania 39. The secondhalf of the cell is provided by wall 43, which may also be polycarbonatebut can be other materials as well. The second electrode 44 is aluminum,platinum, or aluminized thin film coating on a substrate, for example. Aseparator membrane 41 is shown, to allow the hydrogen and oxygen gassesreleased in photoelectrolysis to be collected separately. Further, thiscontrols the amount of dissolved oxygen that is present in the water, tomake the photoelectrolysis reaction more efficient and predictable. Anoptional bias voltage source 45 is shown connected to the electrodes toadjust the redox potential for best electrolysis efficiency, but aredox-mediating electrolyte can also be used to reduce hole/electronrecombination if necessary. Reservoirs 46 and 47 collect the separatedhydrogen and oxygen gases.

In FIG. 6., one method of manufacture is shown in which a polycarbonatesubstrate 51 is delivered from roll 50 into an embossing machine, inwhich a stamper 54 containing the desired undulation shape and pattern54′ and wrapped around roller 53, is embossed into the polycarbonateusing known embossing techniques that may include heat and/or solventsto varying degrees. The stamper is typically a nickel replica grown froma master. The master is typically a photoresist or photoablative polymeron a glass or silicon substrate, into which has been formed the desiredundulations by one of the following methods: contact lithography,projection lithography, interferometric lithography, or laser beamrecording. Finally, the embossed polycarbonate web is coated in a vacuumchamber 55 with e-beam 56 (preferred method), sputtering, ion-assisteddeposition, or thermal evaporation, from a target 57. Material 58 fromthe target then deposits onto the web. The result is that thepolycarbonate emerges from the chamber coated with, in this example,titania 59. In this simplified drawing, provisions 60 and 61 are shownto maintain vacuum lock on the web, but the entire roll can be in thecoater as well. Sol gel or chemical vapor deposition are also possiblemeans of coating. Also, instead of roll manufacture, the polycarbonatesubstrates can be injection/compression molded to the stamper. All ofthese techniques are known in the optical data storage and otherindustries, but their application to this invention is new. The factthat bandgap-shifted titania can be manufactured with existinginfrastructure in low-cost mass production methods enhances the value ofthis invention, because any solar energy conversion application requireslarge area devices with low-cost.

FIG. 7. is a cross-sectional illustration of a multi-layer device 79,comprising individual cells 71, 72, and 73, in which the conductiveanode or cathode is not aluminum or platinum, but transparent indiumtin-oxide (ITO) 70. In this way, a single web 66 is coated with ITO 70on the incident flat side and TiO₂ 68 on the corrugated anode side 67,and then this web is cut up and layered so that the water 69 can flow,or at least wick or capillary, between the layers. The final bottomsurface 75 is metallized with aluminum 74 or other high-reflectancemetal. Also, the edges of the films may be connected electrically withaluminum or platinum or other metallized mount, although that is notshown here to preserve clarity. Photoelectrolysis occurs when light 65from, for example, the sun, illuminates the cell. Means for collectingthe hydrogen gas are not shown here.

In another embodiment, the TiO₂ corrugated sheet anodes are arrangedvertically in parallel in a tank or cell, with the light coupled in froma common concentrator via total internal reflection wave-guiding (edgeillumination) within the anode. FIG. 8 shows the same multi-layer device79 but arranged to be vertical in a tank 83 of aqueous solution 84, andfor which the light 80 required for photoelectrolysis is focused orconcentrated by lens 81 into a cone 82, which then couples into the edgeof the multi-layers and travels down the titania surface via opticalwave-guiding. Lens 81 is intended to be generally representative oflight concentrating optics, and can also be a Fresnel lens or a mirror,for example.

FIG. 9 is a cross-sectional view of a solar photovoltaic cell 95 thatuses titania whose bandgap has been shifted into the more abundant partof the solar spectrum, by employing stress as taught in this invention.Light 90 from the sun is incident onto a polycarbonate cover 91, whichhas the stress-inducing undulations. A transparent indium tin oxide(ITO) conducting layer 92 is first applied to the polycarbonate, whichis then followed with the titania layer 93 and conducting backplane 94,producing voltage V 96.

Also within the scope of this invention is the use of this samephotocatalytic effect, combined with and enhanced by our template grownphotocatalyst with stress-induced band shifting, for the application ofdetoxification and/or disinfection. In these applications, shown in FIG.10., harmful toxins or germs and bacteria 104 are reduced to harmlesscompounds 105 through oxidation by the following process: when a photon100 of the required energy strikes the titania (TiO₂) 103, an amount ofenergy equal to the bandgap of the semiconductor is absorbed. Thisresults in an electron from the valence band of the semiconductor beingpushed up into the conduction band resulting in formation of anelectron-hole pair. The hole accepts an electron from an adsorbed OH⁻ion resulting in the formation of OH⁰, i.e., an OH radical. This freeradical is an extremely powerful oxidizing agent, and can oxidize mostorganic compounds that come into contact with the photocatalyticsurface. With the titania bandgap lowered through this invention, i.e.undulations 102 in polycarbonate substrate 101, detoxification and/ordisinfection will occur more efficiently in visible light, whetherartificial or sunlight.

FIG. 11A. illustrates an extension of the invention in which thesubstrate is a polymer fiber 110 with circular cross section 112 coatedwith titania 111. And FIG. 11B illustrates an extension of the inventionin which the substrate is a polymer sphere 115 with circularcross-section 114 coated with titania 113. In both cases the titaniathin film, because of the curvature of the substrate and the thermalmismatch between the substrate and the titania film, is highly stressed,as in the case of the corrugated substrate before. In FIG. 12, amultitude of titania-coated polymer fibers 121 and 122 are woven orotherwise assembled into a fabric 120 for protective clothing, in whichthe bandgap shift is used for photocatalysis, disinfection, ordetoxification. Or, the fibers can be woven into filters for air andwater purification, or into paper for, say, envelopes that are easilyand effectively disinfected in the presence of light. Similarly, thetitania-coated polymer spheres 126 can be assembled into a filter mesh125, or can be sprayed or otherwise applied to clothing and othersurfaces for passive disinfection and detoxification in the presence oflight.

There are other benefits that this photoelectrolytic surface, with itsnano-optics formed by and embedded into polymer surfaces, brings to anyconversion of solar energy.

First, the cost is low because the technology exists now for embossingand coating acres of polymer web at very high rates of speed. Thesemiconductor material is very thin and therefore contributes verylittle to the cost. The simple one-layer structure cathode, without moreexotic and costly semiconductors, also keeps down the cost, althougheven multilayer semiconductor structures would still be inexpensive withthis method.

Second, this process easily makes continuous large sheets with no “dead”areas, and in solar conversion detector area is paramount.

Third, the focal surface is corrugated, so that the effective activearea is even larger than the projected footprint area, by a factor ofabout 1.4× for sinusoid cylinders, and 2× for hemispheres.

Fourth, the corrugated substrate causes the film to be more robust byproviding mechanical locking, and so prevents the cracking, crazing, anddelamination common to other coating of plastic, and allows the film toexist in a highly stressed state.

Fifth, there is no need for tracking mechanism because the nano-opticshave a large angular field of view, and can keep the sun focused on theinterface over much of the day.

Sixth, the materials are not toxic, and have long lifetimes if a U.V.resistant polymer is chosen.

Seventh, the substrate is very thin and pliable, and is easily rolled upinto a compact volume for unfurling later in space deployment, forexample.

Eighth, the concentrated light makes for better performance of thesemiconductor under low light conditions, where normally low lightconditions allow the electron and hole pairs to recombine. Yet, theconcentration, in the vicinity of 10 suns, is not so much as to causecharge saturation.

Ninth, the corrugations can be designed with a pitch to wavelength ratiofor which light at that wavelength is very efficiently absorbed, as inphotonic bandgap crystal-type anti-reflection coatings, for higherefficiency. In this case, the pitch of the surface template profile isdesigned to increase solar absorption and decrease solar reflection. Thegeometry can then considered to be a 2D photonic bandgap crystal. Also,the pitch, when sub-wavelength, causes very low scatter loss.

Alternately, the thickness of the titania itself can be chosen to be aquarter wavelength antireflection filter for the predominant wavelengthof the bandgap. If stress is not sufficient for this thickness, thetemplate profile or deposition conditions or substratethermal/mechanical coefficients may be altered accordingly.

Devices utilizing this photoelectrolytic surface provide hydrogendirectly for the coming hydrogen-based energy world, and also provide away to convert solar energy into a form that can be stored, i.e. in theform of hydrogen. In addition, the simple and low-cost implementation iswell suited to help energy-impoverished third world countries.

Finally, the clean desalinated water that results from local powergeneration with fuel cells fueled by the photoelectrolysis can be usedfor crop irrigation and other human consumption.

While the primary illuminant considered to this point has been the sun,and the primary benefit the use of free sunlight to passively producehydrogen gas fuel, clean and desalinated water, and detoxification, viastress-induced bandgap-shifting of titania, in particular, to bephotoactive in the solar spectrum, there is merit in using otherilluminants as well.

For example, for the application wherein stressed titania is woven intofabrics, envelopes, and other surfaces for detoxification anddisinfection, artificial illuminant sources that are rich in blue lightbut less so in ultraviolet, such as xenon flash lamps and xenoncontinuous light sources, are more efficiently used than with titania inan unstressed condition.

Another important artificial light source is the blue laser diode.Lifetimes of blue laser diodes have improved to commercial levels, andtheir brightness has increased, while costs have gone down. Shorterwavelength blue lasers, however, still have lifetime, brightness, andcost problems. And ultraviolet laser diodes do not yet exist. Combiningthe blue laser diodes with the present invention, however, makespossible point-of-use photoelectrolysis. FIG. 13 shows a schematic forpoint-of-use photoelectrolysis, in which a blue laser diode 135, ordiodes, illuminates a stress-induced bandgap-shifted titaniaphotocatalyst 136, such that a small injected stream of water or otheraqueous solution 131 delivered by fuel pump 132 from fuel tank 130 isdissociated into hydrogen and oxygen gas by the laser-illuminatedtitania 136, where it can be used directly in the cylinder 137 of aninternal combustion engine, shown partially as 138. Ignition is providedby spark plug 139 and high voltage 140, with the resulting combustion ofthe hydrogen in the cylinder drives the piston 138′. The energy for thelaser diode can be from batteries recharged by braking, as is done todaywith hybrid electric/combustion cars, or just from the alternatorrecharging if the efficiencies work out to allow this. A holding tank134, or “capacitor,” would likely be an advantage and would allowsufficient hydrogen to build up before release into the cylinder, orelse additional laser diode and titania photocatalyst combinations couldbe employed.

A detailed view of one way to combine the photodiode and the titaniaphotocatalytic surface is shown in FIG. 14. Water 141 is injected intoand between titania photocatalytic surfaces 145 and 147 on substrates144 and 146, respectively. Blue laser diode 142 and 144 illuminate thetitania, activating the photoelectrolysis of the water. The resultinghydrogen 149 is injected into reservoir tank 148, or directly into thecylinder. Such a point-of-use hydrogen production could also be usedwith gasoline as the source of the hydrogen. Further, the hydrogen couldbe used to run fuel cells for electric vehicles, rather than forcombustion engines. In any case, on-board storage of hydrogen, andlosses associated with release of hydrogen from such storage means asmetal hydrides, for example, are eliminated.

To this point, the stress induced in the titania layer is largely staticand intrinsic to the substrate. FIG. 15 shows an application in whichthe substrate is a piezo crystal 151 and 160. Application of voltage V155 and 158 causes the piezo crystal substrate to grow by a controlledand calibrated amount 161, which in turn causes tensile stress 162 inthe titania layer 164 that is coated onto the undulating substrate 163that is laminated to the piezo crystal, or in titania layer 152 that iscoated directly onto the piezo crystal 151. Both devices shown in FIG.15 will direct incident light 153 and 156 differently according to theamount of stress applied and therefore the amount of bandgap shifting,so that light 154 and 157 is redirected in a controlled manner. Suchdevices can be used for switching and tuning in photonic applications,including telecommunication network devices.

In summary, this invention provides for shifting the optical bandgap ofa semiconductor into longer optical wavelengths by stressing thesemiconductor, where the semiconductor is a thin film, and where thestress is caused by some or all of the following: conditions under whichthe thin film is coated, the shape of the substrate on a nano, micro, ormacro scale, and the mechanical, chemical, and thermal properties of thesubstrate. The self-focusing of the illuminant also creates localheating, which also serves to shift the bandgap into longer wavelengthswhich are more abundant, for example, in the solar spectrum. Titania isthe preferred semiconductor photocatalytic embodiment, but the inventionapplies to any semiconductor that is photo-active, such as silicon,germanium, and their alloys, and compounds that include, in addition,gallium. The stress-inducing template profiles also provide a mechanicallock to the coating so that the stress can exist without causingdelamination of the coating from the substrate. In addition, thestress-inducing template profiles create additional surface area withoutincreasing the footprint area of the surface, which creates higherefficiency in photocatalytic, photovoltaic, and photoelectrolyticaction.

The source of hydrogen for the photoelectrolysis to act on can be water,sea water, an aqueous solution with electrolytes, or otherhydrogen-bearing liquids such as gasoline.

The illumination is from the sun, the illumination is from artificiallight, the illumination is further concentrated by the stress-inducingtemplate shape by self-focusing, the concentration of the illuminationis largely independent of incident illumination angle, thereby reducingor eliminating the need to track the sun in the sky, the stress-inducingprofiles in the substrate may be one-dimensional or two-dimensional, thepitch of the stress-inducing profiles may be chosen relative to thedesired illumination wavelength such that absorption is increased andreflection is decreased as in a photonic bandgap crystal, the thicknessof the titania layer is chosen to be ¼ of the wavelength of the desiredillumination, thereby acting as an anti-reflection filter and increasingabsorption and decreasing reflection, the substrate surface profiles arechosen to be a certain shape, depth, and radius so that the titania filmgrows as lenses over the profiles, the thickness of the titania ischosen so that the focal plane of said lenses is coplanar with thedistal surface of the titania layer, the additional effective surfacecreated by the substrate stress-inducing profiles facilitates andimproves heat dissipation, the semiconductor is vacuum coated onto orinto the shaped substrate, the semiconductor is applied as a sol gel,the semiconductor is applied with chemical vapor deposition, thesemiconductor is a contiguous film, the semiconductor is a matrix ofparticles such as spheres, the substrate can be polymer, glass, silicon,stainless steel, copper, aluminum, or substrate material, thephotocatalyst is used to detoxify a medium in contact with it, thephotocatalyst is used to disinfect a medium or biological agent incontact or proximal with it.

The substrate is transparent, the substrate is reflective, the substratecan be flexible, the substrate and titania coating are compatible with aroll-to-roll web manufacturing process, the substrate profiles areembossed into the substrate with a master, the substrate profiles aremolded into the substrate, the substrate profiles are caused byreticulation in the substrate or in a layer applied to the substrate.

The titania-coated substrates can be stacked in layers to increaseefficiency for a given area, and said titania-coated stacked substratesmay be pre-coated on the opposite side with a transparent conductingelectrode such as indium tin oxide (ITO), the titania-coated substratesare edge-illuminated, the semiconductor is strontium titanate (SrTiO₃),amorphous silicon, or other semiconductor, the titania-coatedsubstrate(s) function as the anode in a photoelectrolytic cell, whichfurther comprises some or all of the following: a housing, an aqueouselectrolyte, a separation membrane, a cathode, and a bias source, wherethe photoelectrolysis converts solar energy into a chemically storableform, e.g. hydrogen, and may be further combined with a metal hydride orother adsorber.

The self-focusing provided by the nano-lens shape of the titania on thecorrugated substrate improve performance in low light levelapplications.

The invention applications include both terrestrial and spaceenvironments.

The illuminant is a laser diode or laser, a spark between electrodes, ora flashlamp.

The hydrogen is produced at point of use by artificial illumination.

The substrate shape is used to increase or otherwise control the stressin the titania film, where the shape can be concave or convex or a mixof both, and the scale of pitch or radius of curvature can be fromnanometers to meters. Or, the substrate is a piezo device such that theamount of stress induced in the titania film, and therefore the bandgap,is tunable over a range, for use in photonics. Or, the substrate istemperature controlled, such that by changing the temperature thesubstrate contracts or expands and causes a tunable bandgap shift in thetitania or other photocatalyst layer. The substrate can also be a verysmall sphere, typically several microns in diameter but as small as tensof nanometers in diameter, and the material is a polymer, glass, metal,or other material, and is coated with titania or other suitablesemiconductor, said sphere is one of many applied to a surface orsurfaces, or are in suspension in a fluid, and can be applied byspraying, painting, or inkjet deposition. The substrate can also be asmall diameter polymer or other fiber, and the titania-coated fiber iswoven into fabrics for protective clothing or into paper for envelopesthat are readily anti-biotic when illuminated with daylight orartificial light, where the application is photovoltaic, and the stressis enabling (titania) or improving (amorphous silicon).

Applications include photoelectrolysis, detoxification, disinfection,point-of-use photoelectrolysis for use in a hydrogen-based internalcombustion engine, water desalination (where the product of combustionof the hydrogen and oxygen gases from photoelectrolysis is desalinatedwater), and point-of-use photocatalyst device is used in ahydrogen-based internal combustion engine, continual tuning of stressand bandgap properties for telecomm devices, and alteration andimproving magnetic properties of thin films applied to hard drive disksfor data storage.

This invention provides a corrugated substrate to which a desiredtitania or other thin film will adhere under stress but will not causescatter or diffraction due to its sub-wavelength spatial period, therebyallowing low temperature deposition onto polymers, and where thesinusoidal interface at the high and low index thin film and substraterespectively further cause an effective index that varies gradually fromone index to the other, with gradient index benefits of improvedtransmission and reduced reflection, The fact that the photocatalyst isa thin film reduces the probability of recombination of the hole andelectron pairs that occurs in bulk semiconductors in the absence of ananode (or cathode) and electrolyte. The titania coating is evaporatedfrom a titania target, a titanium target with oxygen bled into chamber,or a Ti_(x)O_(y) target such as Ti₂O₃.

In addition, the titania coating may comprise rutile and/or anataseand/or other polymorphs, as well as amorphous titania. Additional thinfilms may be applied between the titania and the substrate in order topromote adhesion or to further modify the stress in the titania.Although the preferred embodiment has been described herein, it will beunderstood that surface features with other dimensions and shapes,substrates of other materials that are not polymers, substrates innon-planar shapes, and other semiconductors (such as SrTiO₃), even thoserequiring a bias voltage, are within the scope of this invention. Forexample, in amorphous silicon solar cells, the use of the corrugatedtemplate/substrate to cause the amorphous silicon film to be undulatingwould create stresses within the silicon layer sufficient to shift,decrease, and broaden the band-gap in the film, and thereby allow moreefficient conversion of the solar spectrum of light into electricity.

While the invention has been described with reference to particularembodiments, it will be understood that the present invention is by nomeans limited to the particular constructions, and methods hereindisclosed and/or shown in the drawings, but also comprises anymodifications or equivalents within the scope of the claims.

1. A photoelectrolytic apparatus for production of hydrogen gas from anaqueous medium, the apparatus comprising: a housing capable of holdingan aqueous medium, the housing permitting ultraviolet and visible lightto enter the interior of the housing; a first electrode, the firstelectrode comprising a substrate having an undulating surface and asemiconductor film on the undulating surface, the semiconductor filmconforming to the undulating surface so as to induce a stress of atleast about 100 mPa in at least part of the semiconductor film, therebyaltering the bandgap of the stressed part of the semiconductor film; asecond electrode; and means for collecting hydrogen gas generated at thesecond electrode substantially free from oxygen generated at thesemiconductor film.
 2. A photoelectrolytic apparatus according to claim1 wherein the undulations on the substrate are substantiallycylindrical, hemispherical, or sinusoidal in profile and have a pitchnot exceeding about 370 nm.
 3. A photoelectrolytic apparatus accordingto claim 1 wherein the semiconductor comprises titania, doped titania ofthe formula nTi_(x)O_(y), or a metal titanate.
 4. A photoelectrolyticapparatus according to claim 1 wherein the housing is substantiallycylindrical.
 5. A photoelectrolytic apparatus according to claim 1wherein the housing is comprised of UV transmitting acrylic,borosilicate glass, quartz, or fused silica.
 6. A photoelectrolyticapparatus according to claim 1 wherein at least part of thesemiconductor film is stressed so as to reduce its bandgap to notgreater than about 3.0 eV.
 7. A photoelectrolytic apparatus according toclaim 1 wherein the substrate comprises polycarbonate or titanium.
 8. Aphotoelectrolytic apparatus according to claim 7 wherein the titanium istubular
 9. A photoelectrolytic apparatus according to claim 1 furthercomprising means for concentrating incident radiation and substantiallyfocusing it on to the titania electrode within the housing.
 10. Aphotoelectrolytic apparatus according to claim 9 wherein the housing issubstantially cylindrical.
 11. A photoelectrolytic apparatus accordingto claim 9 wherein the housing is comprised of one or more of UVtransmitting acrylic, borosilicate glass, quartz, and fused silica. 12.A photoelectrolytic apparatus according to claim 9 further comprising aphotovoltaic strip having a length approximately equal to the housinglength and a width substantially equal to or less than the width of thehousing and a spectral filter of substantially the same dimensions asthe photovoltaic strip, the spectral filter being placed between thetitania electrode and the concentrating means, such that a first part ofthe incident radiation passes through the filter and on to the titaniaelectrode, but a second part of the incident radiation is reflected bythe filter on to the photovoltaic strip, the negative terminal of thephotovoltaic strip being connected to the second electrode, and thepositive terminal being connected to the titania electrode, such thatelectricity produced by the photovoltaic strip upon illuminationprovides a bias voltage to the photoelectrolytic cell for increasedhydrogen production.